1. Field of the Invention
The present invention relates to a sensor for measuring the flow rate of a fluid or, in particular, to a flow rate sensor employing a diaphragm.
2. Description of the Related Art
As a means for measuring the flow rate of a fluid, a system in which the pressure of each of two pressure sensors arranged with an orifice held therebetween is calculated, is generally known. This system, however, poses a great problem in respect to sensitivity and pressure resistance. Specifically, in the case where the portion of the system downstream of the flow rate sensor is closed, a large pressure is imposed and, therefore, a pressure sensor having a high pressure resistance is required, resulting in a lower sensitivity. Also, the use of two sensors leads to different drift characteristics (temperature, source voltage, etc.) due to the individual differences of the pressure sensors, which in turn poses the problem that correction and the zero adjustment, after a calculation, are frequently required.
In view of this, the present inventor previously proposed, in U.S. Pat. No. 3,184,126, a flow rate sensor used for ultra pure water and chemical liquids which are used, for example, in the semiconductor manufacturing process. This flow rate sensor comprises a first diaphragm and a second diaphragm for defining a primary chamber and a secondary chamber which are connected by a bypass with an orifice interposed therein, wherein the displacement generated by the pressure fluctuations of the fluid received by the first and second diaphragms is detected by a strain gauge.
The patented system described has several great advantages. For example, it does not have any movable member (such as an impeller or a float) for detecting the flow rate in the flow path of the fluid to be measured, and therefore no fine dust (particles) is generated. Thus, the system can be most suitably used for measuring the ultra-pure water or chemical liquids. In addition, the flow rate change can be detected directly as an electrical signal, thereby making it possible control the subsequent operation easily. Especially, in view of the fact that an orifice (portion) is interposed in the bypass, the diameter of the orifice can be set arbitrarily and minimized for detecting a very small flow rate. Also, the orifice, if formed as an independent member, can be replaced freely in accordance with the diameter thereof.
In the structure of the patented invention described above, however, the sensitivity of the load difference sensor (strain gauge) must be improved for measuring a fine differential pressure in respect of the flow rate. Reducing the thickness of the movable member of the load difference sensor for improving the sensitivity thereof, however, would erroneously exert a high pressure on the primary or secondary side. Also, in the case where an excessive flow rate exceeds the measurable range, the limit of elasticity of the movable member of the load difference sensor is exceeded, thereby damaging the movable member or making it impossible to restore the zero point. Increasing the pressure resistance of the movable member of the load difference sensor, on the other hand, would reduce the sensitivity against the differential pressure in respect of the flow rate, with the result that a wide range of the measurement ability of the flowmeter cannot be secured. Thus, the accuracy would be adversely affected without allowing a large pressure loss in the chamber by reducing the orifice diameter or otherwise.
In view of the situation described above, the object of the present invention is to provide a novel structure of a flow rate sensor which has a high pressure resistance while at the same time facilitating the measurement of the differential pressure for measurement of a very small flow rate.
According to a first aspect of the invention, there is provided a flow rate sensor comprising a chamber with first and second diaphragms arranged therein in opposed relation to each other, wherein the chamber is divided into a primary chamber facing the first diaphragm and a secondary chamber facing the second diaphragm, wherein a fluid is passed from the primary chamber to the secondary chamber through a bypass having an orifice member thereby to generate a differential pressure between the primary chamber and the secondary chamber, wherein the load difference generated by the fluid pressure fluctuations received by the first diaphragm and the second diaphragm is detected as a displacement by a load difference sensor arranged between the first diaphragm and the second diaphragm thereby to detect the flow rate of the fluid, and wherein a selected one of the diaphragm pair and the load difference sensor includes at least a displacement limiting member for preventing the displacement generated by the fluid pressure fluctuations received by the diaphragms from increasing beyond a predetermined amount.
According to a second aspect of the invention, there is provided a flow rate sensor of the first aspect, wherein the load difference sensor includes, fixedly arranged in the chamber, an outer peripheral frame member, a central member arranged at the central portion of the chamber, and a deforming portion, with a measuring portion, extending between the outer peripheral frame member and the central member, and wherein a pressure receiving portion for receiving the pressure of each of the diaphragms and transmitting the load thereof is mounted on the central member of the load difference sensor.
According to a third aspect of the invention, there is provided a flow rate sensor of the second aspect, wherein the displacement limiting member is arranged inside each of the pressure receiving portions.
According to a fourth aspect of the invention, there is provided a flow rate sensor of the second or third aspect, wherein a protective diaphragm portion is formed outside the deforming portion having the measuring portion of the load difference sensor.
According to a fifth aspect of the invention, there is provided a flow rate sensor of the second or third aspect, wherein the load difference sensor includes two deforming portions formed in opposed relation to each other and a measuring portion is arranged inside each of the deforming portions.
Further, according to a sixth aspect of the invention, there is provided a flow rate sensor of the first aspect, wherein an inlet portion and an outlet portion for the purge gas in communication with the space adjacent to the back sides of said diaphragms is arranged in the body of said flow rate sensor, and the permeative gas existing in the space adjacent to the back sides of said diaphragms is discharged outside together with the purge gas passing therethrough.
Furthermore, according to a seventh aspect of the invention, there is provided a flow rate sensor of the sixth aspect, wherein means for detecting the gas concentration or the liquid leakage is arranged in the piping system on the outlet portion side of said purge gas.